This type of problem is often caused by a contaminated laser lens. You can attempt to clean it by blowing out the disc tray/slot area with a can of compressed air for dusting computers (keep the can upright only so you don't get chemical propellant on the lens).

If neither of these cleaning steps help, then this is more likely a hardware fault, which could include a bad laser emitter or pickup, faulty spindle motor or laser transport, bad d/a converter, etc. Any of these issues would require repair/replacement.